Optimal nutrition for the very low birthweight infant has yet to be established. Current recommendations for weight gain are based on in utero accretion rates without knowledge of exact energy requirements in these infants. Additional stressors are placed on both the ill preterm and term infants above their stable counterparts. Because of this, there has been a general consensus that the energy expenditure of these infants is correspondingly increased. Current studies to evaluate this issue have focused on older chronically ventilated infants rather then infants in the acute stages of illness. While some studies suggest a trend toward increased metabolic requirements, others suggest that current methods of calculating energy expenditure are inadequate and may be overestimating needs. We propose to measure total energy expenditure in term and preterm infants using the double labeled water method. We hypothesize that the energy expenditure of ill preterm and term infants will be greater than that of their stable counterparts. We hope to more accurately assess the energy expenditure of these infants and with the information obtained structure more appropriate recommendations for energy intake based on gestational age and presence or absence of illness.